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Middle and Upper Jurassic

Chronostratigraphy Middle Jurassic rocks reflect a variety of depositional environments including shallow marine, fluvial, deltaic, saltmarsh and coastal lagoonal (brackish-water and freshwater). In addition, carbonate (limestone) as well as clastic (mudstone, siltstone, sandstone) sedimentation was often widespread. These two factors – depositional environments and increased carbonate sedimentation – are largely responsible for the distinctive characteristics of the Middle Jurassic succession in Britain which, is in contrast to the marine mudstone-dominated Lower and Upper Jurassic successions. Middle Jurassic rocks crop out in an almost continuous strip from the Dorset coast to the North Yorkshire coast. In Scotland, Middle Jurassic rocks crop out in the inner Hebrides and NE Scotland.

Stage subdivision in the Middle and Upper Jurassic System is by means of ammonites, although Middle Jurassic ammonites are rare or absent from many intervals because of unfavourable depositional environments (e.g. Aalenian–Bajocian strata in the Cotswolds or Bathonian succession in northern England and Scotland), it is not possible to apply an ammonite biozonation. Consequently, the standard ammonite-based standard zones are identified, wholly or in part, by indirect or circumstantial evidence (e.g. ostracod faunas, event-marker correlation). Late Jurassic ammonites have been used to develop high resolution zonations and subdivisions with the development of sophisticated schemes of stratal subdivision and correlation, with ammonite faunal horizons or biohorizons being the highest ammonite stratigraphical refinement.

In the Middle Jurassic seas, in which limestones formed, calcareous seaweeds (red algae) were common. Invertebrate faunas living on the seabed included simple and compound corals, calcareous sponges and bryozoa, which sometimes formed small patch reefs. Bivalve molluscs burrowed in the soft sea-floor sediments, and some surface-dwellers (for example oysters) built up shell reefs. Smooth terebratulid and ribbed rhynchonellid brachiopods occurred in an abundance that was never repeated on such a scale in younger geological times. Lobster and shrimp-like crustaceans dwelt in both muddy and carbonate seabed environments, leaving characteristic burrow systems. Belemnites and fish were the main food of aquatic reptiles, which included ichthyosaurs, plesiosaurs, pliosaurs, crocodiles (steneosaurs and teleosaurs) and turtles. On land, dinosaurs, for example Cetiosaurus, were vegetable feeders, while others, such as Megalosaurus, were flesh-eaters. Crocodilians, lizards and amphibians (such as frogs and salamanders), were also part of the terrestrial fauna. Land plants in Mid Jurassic times were dominated by the gymnosperms, notably conifers, cycads and ginkgoes.

By the beginning of the Oxfordian, a shallow shelf sea was established over much of Britain, with sea levels rising through the Oxfordian with consequent widespread deposition of mudrocks. By the end of early Oxfordian times shallower water deposits of limestones and sandstones of the ‘Corallian facies’ developed. The Corallian Group is the best known part of the Upper Jurassic because it forms hilly ground from Dorset to Oxford, to North Yorkshire. In the Oxfordian, corals became a significant part of the marine fauna where there was carbonate sedimentation. Sponges, bryozoa, echinoids, gastropods and a prolific bivalve fauna also lived on or in these limey sediments. Microscopic organisms such as foraminifera and ostracods and phytoplankton (such as dinoflagellates, coccolithophorid algae) flourished in the warm seas.

Mudrocks are the predominant lithology of the Kimmeridgian, which represents a period of globally high sea level and were more extensive than any previous Mesozoic deposit. The extent of previously established landmasses shrank during the Oxfordian and Kimmeridgian ages) in response to the rising sea levels, . This high sea-level was terminated by a major fall in global sea level in mid Tithonian (Portlandian) times, leading to the re-emergence of land areas such as the London Landmass, and locally the deposition of carbonate shallow marine shelf systems, particularly in the Wessex basin (extracts from Cox & Sumbler, 2002; Wright & Cox, 2001).
MWH
Further Information
Cox, B.M. & Sumbler, M.G., (2002), British Middle Jurassic Stratigraphy, Geological Conservation Review Series, No. 26, Joint Nature Conservation Committee, Peterborough, 508 pages.

Wright, J.K. & Cox, B.M., (2001) British Upper Jurassic Stratigraphy, Geological Conservation Review Series, No. 21, Joint Nature Conservation Committee, Peterborough, 266 pages.